1. Field of the Invention
The present invention relates to a centrifugal compressor, and more particularly, to a centrifugal compressor provided with diffusers which is required for an enlarged operation range and a high efficiency.
2. Description of the Related Art
In the centrifugal compressors, the flow of a gas or vapor is generally distorted in a widthwise direction at the exit of a centrifugal impeller. This tendency is evident in a compressor whose ratio of compression per stage is larger than 2. The gas flow is distorted such that the direction thereof is close to the tangential direction of the impeller on the side of a front shroud and deviates from the tangential direction on the side of a main shroud as compared with that of the gas flow on the side of the front shroud.
In such a compressor, an increase in the flow rate results in choke in the minimum cross-sectional area of a diffuser formed by the stator blades and the side walls and therefore thus decreases the performance of the compressor suddenly. Furthermore, under the operation conditions in which the rotational speed is relatively high and in which the flow rate is low, separation occurs on a suction surface of a stator blade, thereby generating surging in which a sufficient pressure increase cannot be obtained. These problems are solved by a diffuser with an auxiliary blade provided between the adjacent stator blades.
In the case where the auxiliary blade is provided in the diffuser, an auxiliary blade which is as thin as possible is desired to prevent an decrease in the minimum cross-sectional area of the flow passage. However, a minimum thickness is necessary in terms of the strength. Hence, the number of stator blades is determined such that it assures a sufficient cross-sectional area of the flow passage. However, this increases the gap between the stator blades near the outer periphery of the stator blades. As a result, separation occurs in a large scale along he suction surface of the stator blade near the trailing edge thereof, and the cross-sectional area of the gas flow, hence, the cross-sectional area of the diffuser substantially thereby decreases while diffusion of the kinetic energy occurs in the separation.
The operation range of the centrifugal compressor of the above-described type is often limited by the diffuser. That is, stalling of the gas flow in which the back flow of the gas generated in the diffuser reaches the impeller determines the minimum flow rate, and choke in the diffuser determines the maximum flow rate. Hence, it is said that the operation range of the compressor can be enlarged by enlarging the operation range of the diffuser, and several techniques thereof have been proposed. In one of such techniques which have been widely used, stator blades are supported in such a manner as to be rotatable about a shaft provided parallel to the shaft of the impeller, and the direction of the stator blades is varied in accordance with the direction of a gas which flows out of the impeller. In another technique, the distal end of the stator blade is extended to form a boundary layer fence (disclosed in U.S. Pat. No. 3,781,128). In another technique, the angle of inlet of the stator blade is varied in the direction of height thereof (disclosed in U.S. Pat. No. 3,904,312, Japanese Utility Model Unexamined Publication No. 53-162405).
However, the conventional compressor of the above-described type in which the stator blades are made movable have a complicated configuration. In the compressor in which the boundary layer fences are provided, the effect of the boundary layer fences cannot be expected sufficiently in the case of the high-speed centrifugal compressor in which distortion of the flow of fluid at the exit of the impeller is large. In the case of the compressor in which the angle of inlet of the stator blade can be varied in the direction of the height thereof, sufficient enlargement of the operation range cannot be obtained. Furthermore, in the compressor in which the leading edge of the stator blade is disposed close to the impeller in order to enlarge the operation range, the stator blade or the impeller may be broken after the operation over a long period of time due to the strong exciting force caused by the aerodynamical interference between the impeller and the stator blade.